Kyrie

Kyrie is a supporting character in Devil May Cry 4. She was born and raised in Fortuna, is the younger sister of the leader of the Order of the Sword's Holy Knights, Credo, and songstress for the Festival of the Sword. Kyrie is the childhood friend of Nero, as well as his lover. She carries a necklace that was given to her by Nero at the beginning of the game as a present during the festival. The love between Kyrie and Nero seems to have started before the events of Devil May Cry 4, but their love is used as a major plot point on several occasions during the game.

Devil May Cry 4
At the beginning of the game she is seen singing an opera, occasionally glancing at the empty seat where Nero was supposed to have occupied. Upon Nero's return, he hands her a gift as the two sit beside each other while Sanctus explains the history of Sparda's rebellion against Mundus and his demon brethren. As the people begin to pray, Dante crashes in and assassinates him, causing a widespread panic which briefly separates Nero from Kyrie as she loses the gift given to her which is quickly trampled upon in the confusion. As Dante approaches her brother Credo, Kyrie is knocked down and cowers in fear before the Devil Hunter before Nero comes to her apparent rescue. After Nero's skirmish with the devil hunter, Kyrie locates the contents of her package: a necklace and goes with Credo as Nero investigates the demon uprising and Dante cases.

Kyrie is not seen for a long while until she happens upon the duel between her brother and Nero and discovers Nero's Devil Bringer arm that he had been hiding from her. Scared and confused, Agnus captures her and flees the scene as Credo attempts to hunt down the scientist. Later on, she is seen captive in some kind of magic seal where Nero attempts to rescue her. As the newly resurrected Sanctus enters the scene to put the final stages of the plan in motion, he takes away Kyrie. Nero fights in a blind rage as he tries to break free from the Bianco Angelos that attempt to stall him. He is almost successful, only to fail and rip away the necklace instead. Kyrie is absorbed into The Savior to be used as a ploy and scapegoat for Nero to lure him in. Nero fights it and the transformed Sanctus and is almost successful in destroying it, but it caught off guard when Kyrie emerges from the statue, allowing Sanctus to defeat him. Credo attempts to kill Sanctus but is struck down before being caught by Dante. Credo makes one final request to save his sister and Nero before his body fades away.

Kyrie is not seen then until the last mission where she is being kept deep inside of the core of the Savior in a fleshy prison. Nero comes to her rescue and promptly defeats Sanctus Diabolica. The madman threatens to kill her if he advances further but catches him off guard when he tosses the Yamato to the false prophet, slams him into a wall with the Devil Bringer, in a swift move with the Yamato, frees Kyrie and impales Sanctus, killing him.. Nero escapes the confines of the Savior with Kyrie safe and sound. Sanctus returns as the False Savior but Nero quickly attends to the matter by pulverizing the twisted visage of the statue.. After settling matters with Dante, Nero reconciles with Kyrie, the latter not caring about his status as a demon or a human, but merely himself. The two almost exchange kisses to profess their love until more demons show up to interrupt the moment. Kyrie confidently waits as Nero attends to the matter.

Background
The Kyrie eleison ("Lord have mercy" in Greek) is the first part of the Catholic Ordinary of the Mass. It is traditionally sung in Greek.

Trivia

 * Of all the lead female characters in the series, Kyrie is the only one who dresses modestly and does not participate in any confrontation or battles with other demons. Despite this, she is featured in the "Ladies of Devil May Cry" bonus image.
 * Stephanie Sheh, Kyrie's voice actress, also plays Orihime Inoue in the anime Bleach. In that same anime, Johnny Yong Bosch, Nero's voice actor, plays Ichigo Kurosaki, the lead protagonist and Inoue's love interest.
 * Kyrie is the only human character in Devil May Cry 4 who can be seen during gameplay, in Mission 9. She can be seen in the cage in which she appears after the fight with Agnus.
 * Similar to this is Vergil in Devil May Cry 3, who can be seen during Mission 7 at the top of the Temen-ni-gru.